1. Field of the Invention
This invention relates to diffusion transfer photographic products and processes employing dye developers. More precisely this invention relates to the use of certain pyridine N-oxides to provide improved processing performance characteristics for such products and processes.
2. Description of the Prior Art
Diffusion transfer photographic processes and products employing dye developers are known to the art and details relating to them can be found in U.S. Pat. Nos. 2,983,606; 3,345,163; 3,415,644; 3,415,645; 3,415,646; 3,473,925; 3,482,972; 3,551,406; 3,573,042; 3,573,043; 3,573,044; 3,576,625; 3,576,626; 3,578,540; 3,579,333; 3,594,164; 3,594,165; 3,597,200; 3,647,437; 3,672,486; 3,672,890; 3,705,184; 3,752,836 and 3,857,855.
Essentially, diffusion transfer photographic products and processes involve film units having a photosensitive system of element including at least one photosensitive layer usually selectively sensitized and integrated with dye developer as a dye image providing material. After photoexposure, the photosensitive layer is developed to establish an imagewise distribution of dye image providing material all or a portion of which is transferred to an image receiving element having an image receiving layer capable of mordanting or otherwise fixing the diffusible dye. The image receiving layer retains the dye image for viewing and in some diffusion transfer products, the dye image is viewed in the layer after separation from the photosensitive system while in other products, such separation is not required.
Multicolor diffusion transfer images may be obtained using dye developers by several known techniques. A particularly useful technique employs an integral multilayer photosensitive element, such as is disclosed in the referenced U.S. Patents, wherein at least two selectively sensitized photosensitive layers, superposed on a common support, are photoexposed and then processed, simultaneously without separation, with a single (common) image-receiving layer. A typical arrangement of this type for obtaining multicolor images utilizing subtractive color principles comprises a support carrying a red-sensitive silver halide emulsion layer, a green-sensitive silver halide emulsion layer and a blue-sensitive silver halide emulsion layer, each emulsion layer being associated, respectively, with a cyan dye developer; a magenta dye developer and a yellow dye developer.
The dye developer may be positioned in the silver halide emulsion layer, for example, in the form of particles, or it may be disposed in a layer behind the appropriate silver halide emulsion layer with respect to the exposing light. Each set of silver halide emulsion and associated dye developer layers may be separated from other sets by suitable interlayers, for example, by a layer of gelatin, polyvinyl alcohol, or other polymeric materials known in the art.
After photoexposure, the photosensitive element is processed by application of a processing composition in manners well known in the art. The exposed photosensitive element may be superposed prior to, during, or after application of the processing composition on a sheet-like element which may include an image receiving layer. Generally, means containing the processing composition and for discharging it within the film unit are employed for applying the processing composition to the photosensitive element in a substantially uniform layer as the photosensitive element is withdrawn from the dark chamber.
The applied liquid processing composition permeates the layers of the photosensitive element to initiate and effect development of the latent images contained there. The dye developers are immobilized or precipitated imagewise in developed areas as a consequence of and in proportion to the silver halide development. At least part of this immobilization is due to a change in the solubility characteristics of the dye developers upon oxidation and especially to a change in the solubility of the oxidized dye developer in alkaline solution. Accordingly, in undeveloped and partially developed areas of the silver halide emulsion layers, the respective unoxidized (unreacted) dye developers are diffusible. Development thus provides an imagewise distribution of unoxidized dye developer, diffusible in the alkaline processing composition, as a function of the point-to-point degree of exposure of a silver halide emulsion layer. At least part of each of these imagewise distributions of unoxidized dye developer is transferred, by imbibition, to a superposed image-receiving layer, with the transfer substantially excluding oxidized dye developer.
The image receiving layer receives a depthwise diffusion, from each developed silver halide emulsion, of unoxidized dye developer without appreciably disturbing the imagewise distribution thereof to provide a reversed or positive color image of each developed silver image. The image receiving layer may contain a mordant and/or other agent to immobilize the dye developer transferred thereto. If the color of a transferred dye developer is affected by changes in the pH of the image receiving layer, this pH may be adjusted in accordance with well known techniques to provide a pH affording the desired color.
As mentioned, the present invention is concerned with dye developer diffusion transfer processes and products in which an N-oxide is present during development. U.S. Pat. No. 3,998,640, issued Dec. 21, 1976 to S. J. Cuirca, Jr. also relates to diffusion transfer film units employing heterocyclic N-oxides as oxidants. According to the patent, the oxidants are particularly useful when employed in film units having oxichromic compounds as dye image providing materials. Oxidation of the dye image providing materials is achieved by using heterocyclic N-oxides having a polarographic reduction potential at least more positive than the polarographic oxidation potential of the dye image providing materials to be oxidized. Particularly useful N-oxides are those having a polarographic reduction potential more positive than -0.5 v. when in an aqueous solution comprising 4% potassium hydroxide.
Representative useful N-oxides disclosed in U.S. Pat. No. 3,998,640 are benzofuroxans and 4,4-axopyridine-1,1'-dioxides of the following formulae: ##STR1## where:
R.sup.1 and R.sup.2 each represent a hydrogen atom, an alkyl group having 1 to 25 carbon atoms, an alkoxy group having from 1 to 25 carbon atoms, a halogen atom, a nitro group, an oxo-linked benzofuran or an organic ballasting group of such size and configuration (for example, simple organic groups or polymeric groups) as to render the compound non-diffusible, especially during treatment with an alkaline processing composition;
n is an integer having a value of 0 to 1; and R.sup.3 and R.sup.4 each represent a hydrogen atom, an alkyl group having 1 to 25 carbon atoms, an alkoxy group having 1 to 25 carbon atoms or an organic ballasting group as described above for R.sup.1 and R.sup.2. Suitable ballasting groups typically contain an alkyl group (branched or unbranched), an aryl group, an aralkyl or an alkaryl group and typically contain 8 to 25 carbon atoms.
According to the present invention, novel diffusion transfer film units employing N-oxides are presented to the art. Unlike the N-oxides of U.S. Pat. No. 3,998,640, the N-oxides employed in the present invention have polarographic reduction potentials less positive then the polarographic oxidation potential of the dye developers of the film units. As will be described in more detail in the description which follows, the use of these N-oxides provides such advantages as better control over dye transfer and particularly better control over magenta dye transfer.